Modelling of Reactors for the Thermal Treatment of Waste and Biomass

Abstract

Modelling of reactors devoted to the thermal treatment of waste and biomass is a very complex task. It has to represent the physical and chemical phenomena occurring at different scales with a feedstock which is often neither completely known nor homogeneous. The first difficulty to overcome is associated to the description of the intrinsic reaction pathway of the pyrolysis reaction (which is at the center of the overall thermal treatment) and its associated kinetics. Indeed, a mathematical model has to be built to compute the rate of consumption of the initial and infinitesimal (but representative) material, the rate of production of the products of reaction as well as their nature. It has to be pointed out here that this first model should be respectful of atom balances during the overall operation. The second point that as to be addressed deals with heat and mass transfer that occur within the particle undergoing reaction. Indeed, as heat is provided to this particle, its temperature increases and when it becomes sufficient, chemical reaction begins (without any other requirement in the case of pyrolysis, or if co-reactant is also present in the case of gasification or combustion) leading to heat and mass production/consumption. These new products must be transported outside of the reactive particle under the effect of internal pressure gradients and diffusion. Then, when they have been released from the particle their future will depend upon their surrounding environment and more precisely upon the nature of the reactor (single particle, fixed bed, fluidized bed…) which will be relevant on the estimation of the extra heat and mass transfer. This short description illustrates the fact that modeling reactors for thermal treatment of biomass requires to describe several phenomena. In this paper, we focus on a multiscale description of the relevant processes involved in the overall transformation. As a starting point, the intra particle scale is considered. Some insights into possible modelling concepts at this step are provided. Then, the particle scale is described. Once again, the important phenomena as well as the modeling options are discussed. Finally, the reactor scale is considered. These previous considerations are applied to the descriptions of model that have been developed in the case of refuse derived fuel grate gasification, combustion of sludge in a fluidized bed and pyrolysis of biomass in a fixed bed.